부산대학교 도서관

The reaction of chlorophyll a/sup +/ (Chl a/sup +/) with either Chl a/sup -/ or pheophytin a/sup -/ (Pheo a/sup -/) in addition to the reaction of Pheo a/sup +/ with Pheo a/sup -/ was studied in butyronitrile (BCN), BCN-1% THF, THF, and DMF. The electrochemically produced radical ion pairs Chl a/sup +/-Chl a/sup -/ and Pheo a/sup +/-Pheo a/sup -/ react in each solvent to produce a 10/sup -7/-10/sup -4/ yield of luminescent states on the basis of the initial number of radical pairs. The Chl a/sup +/-Pheo a/sup -/ reaction produces no observable luminescence in any of the solvents examined. The luminescence maximum for the Pheo a/sup +/-Pheo a/sup -/ reaction occurs at 730 nm in each solvent and is strongly red-shifted relative to the fluorescence maxima for optically excited Pheo a in these solvents. A similar result is obtained for the Chl a/sup +/-Chl a- reaction in BCN. However, emission from the Chl a/sup +/-Chl a/sup -/ reaction in the other three solvents occurs at 680 nm and corresponds more closely to normal fluorescence from optically excited Chl a. The red-shifted spectra are consistent with the formation of excimers. The ac voltammetry of Chl a in BCN provides evidence that Chl a is aggregated in the ground state in this solvent. Chl a reduction shows four waves in BCN and two waves in the other three solvents. Thus, the Chl a/sup +/-Chl a/sup -/ reaction in BCN does not form a true excimer, whereas the Pheo a/sup +/-Pheo a/sup -/ reactions in each solvent do. The luminescence efficiencies of these charge-transfer neutralization reactions are discussed in terms of the geometric constraints on electron-transfer reactions in photosynthetic reaction centers. 4 figures, 3 tables.